Looking for advice on this cascading tankless layout

CDecker

Heat loss calculations were performed using RadiantWorks Pro. Heat source will be (2) Takagi T-H3-DV-N Condensing Tankless water heaters, cascaded or "Easy-Linked" together. They will provide domestic hot water to a large residence, as well as floor warming, and snowmelt. Primary loop and heaters will consist of city water, secondary loop will contain 50% prop. glycol mix. Head and flow are posted in the diagram.

I was hoping someone could double check my pipe sizes? I want to make sure the primary and secondary pipe sizes are correct. The 100 plate SS Heat Exchanger is rated for 400,000 BTU and has 1.25" ports.

Zone pumps will be manually activated and run 24/7. Primary pump(s) will turn on / off based on return water temps and signal(s) from the zone Aquastats.

(2) SS Taco 0013's were selected, in series, to provide enough pressure (40 psi) to produce maximum flow of the tankless units (10 GPM).

Do I need all of these expansion tanks?

My diagram was constructed based off a diagram provided to me by the manufacturer (Takagi) for this application. I have provided the diagram they sent as well.

I appreciate any and all feedback. Thanks!!

-Chris

Zman

You are going to great extremes to use a product for a purpose that it was not intended. Is there a reason you would not use a condensing boiler for this?

Zman

HWH for boiler will wear out quickly
HWH will not condense at high temps.
No way to prioritize DHW
No mixing on DHW
What controls sm and rad temps? Aquastat, as placed, is measuring what?
Head for rad and sm not calced correctly
Extra ex tank not needed.
How was HX sized?

This is the quick list, there is more.

I assume that cost is the reason for tankless.
Have you priced a single 199,000 condensing boiler with indirect? You would eliminate mixing, 2-0013, hx, exp tank, air sep.....

mattmia2

Look at HTP-UFT boilers, I think they will do what you are trying to do without spending 3x what you spent on the DHW heaters on heating specialties to make them do what they were not intended to do. The Pioneer is a nice solution if you have a lot of small zones and need some thermal mass to even out short cycling.

mattmia2

Also, find the calculator for the HX you plan to use, if you go that route (I would go with an indirect off a mod con boiler instead of trying to heat the building with your DHW), the HX performance depends on your supply temps and flow rates, the capacity they state in the marketing is at just ones specific point of that multi dimensional calculation.

mattmia2

Oh, and you will need 400 ft^3/hr out of your gas service just to serve that which means you likely will need to upsize the service since a standard meter is 250 ft^3/hr.